The heavier vehicle, which the DOD has put on a fast track for deployment by 2008, is a replacement to the armored humvee. The military wants it to provide better protection to those inside it, get better fuel efficiency, have more electrical generation capacity and be built from the ground up for this purpose. Plans on the drawing board at Georgia Tech include an "Ultra Armored Patrol" vehicle which would feature a "blast bucket" to protect the crew even if the vehicle itself was lost in an attack or to a bomb. The military has an armored patrol vehicle (the ASV), but they cost about $800,000 each, compared to about $150,000-$200,000 for an armored Humvee. Presumably, a replacement armored Humvee would be made cheaper and more fuel efficient by providing less protection outside the crew compartment.
The lighter vehicle, I have described at dkospedia:
In anticipation of the V-22, the Marines have also begun a program to design a ground vehicle that the V-22 and its CH-47 helicopters can carry. These aircraft are too small to carry a Humvee (the standard military utility vehicle) or heavier armored vehicles. Currently, tests have been conducted using World War II era jeeps and small commercial pickup trucks. The proposed vehicle, currently called the ITV, would be unarmored but able to carry 3-4 Marines and their supplies and heavy weapons a substantial distance (comparable to a civilian motor vehicle) over fairly rough terrain for a wheeled vehicle. Some prototypes resemble big dune buggies. About 2000-4000 ITVs are expected to be purchased.